The present invention pertains to a method of hydrating and releasing a contact lens from the mold section in which it was molded.
Soft contact lenses are produced by a number of methods including static cast molding, spincasting, and combinations thereof. Two-part molds are commonly used in cast molding procedures, whereas spincasting involves the use of a single mold. In all cases where the lens is cast in a mold, it is important to recover the lens without damaging the lens.
As an example, U.S. Pat. No. 5,271,875 discloses a method of cast molding lenses in a mold assembly composed of anterior and posterior mold sections. The method involves filling the anterior mold with monomer mixture, assembling the anterior and posterior sections, clamping them together, curing the monomer to form a lens, disassembling the mold sections, recovering the lens, and hydrating the lens to form a hydrogel lens. This patent also discloses that it is advantageous to ensure that the lens is selectively retained on one of the mold sections.
U.S. Pat. No. 5,271,875 illustrates two general methods of recovering the lens. In both methods, one of the anterior and posterior mold sections is removed, with the lens remaining with the other mold section. First, in a "dry release" process, the lens is disengaged from the mold section in which it is retained by a force applied by a mechanical fixture. Second, in a "wet release" process, the lens is hydrated to facilitate its removal from the mold section in which it is retained. More specifically, as the lens absorbs water, the cured lens material expands sufficiently to disengage from this mold section.
U.S. Pat. No. 5,264,161 also discloses a "wet release" method of recovering a lens. In this method, a contact lens and the mold in which it is retained (preferably the lens being retained in both the anterior and posterior mold sections) is added to an aqueous bath, whereby the lens is hydrated and released from the mold. U.S. Pat. No. 5,264,161 further teaches the advantage of adding a surfactant to this aqueous bath to facilitate release of the lens.
A drawback of dry release methods is that unless proper adjustment and operation of the mechanical release fixture is maintained, there is a high likelihood that the lens will be damaged. A drawback of wet release methods is that they may not adequately disengage the lens from the mold surface, thus requiring manual handling or resulting in damage to the lens when attempting to remove the lens from the mold.
In cast molding lenses, it is common that, in addition to the cured lens, there occurs excess cured lens material. This results from excess liquid monomer mixture being expelled from the lens cavity into an overflow. This excess monomer is then cured along with the lens. Frequently, this excess cured lens material has the form of a ring, formed around the cured lens. It is important that this excess cured lens material is separated cleanly from the lens. In the wet release process such as U.S. Pat. No. 5,264,161 employing an aqueous bath where the excess lens material is hydrated along with the lens, it becomes necessary to ensure that this excess cured lens material does not adhere to the hydrated lens. This is a particular problem since the lens and excess lens material tend to adhere to each other when hydrated.
This invention provides an improved process for hydrating and releasing a contact lens from the mold in which it is retained, offering less occurrence of damaged lens and ensuring that the lens is recovered separately from any excess cured lens material.